Falls are a leading cause of morbidity in the elderly, resulting in 36 million hospital visits per year, at a cost in excess of $10 billion. While the cause of many falls remains unexplained, syncope and orthostatic hypotension are important factors. Since vestibular activation has been shown to have an active role in the regulation of blood pressure during orthostatic stress, and vestibular function decreases with age >60 years, we hypothesize that vestibular hypo-function in the elderly results in impaired regulation of blood pressure during orthostasis, which in turn results in cerebral hypo-perfusion and syncope. Previous evidence that vestibular activation has direct effects on cerebral blood flow suggests that vestibular inputs affect the cerebrovascular response to orthostasis. The goal of this research is to examine the role of vestibular inputs in cerebral blood flow regulation and the effect of these inputs on orthostatic tolerance. Our general hypothesis is that otolith-mediated vestibular inputs act as a feed forward mechanism causing cerebral vasodilatation, to compensate for the decrease in cerebral perfusion pressure caused by the upright posture. We propose two specific aims to address this hypothesis: 1) Determine the effect of tilts in the pitch (sagittal) and roll (lateral) planes on cerebral blood flow and cerebral auto-regulation in young and older subjects (with and without vestibular deficits). This aim will be accomplished by passively tilting subjects with eyes closed from a horizontal to 80 degree upright posture in the pitch and roll planes; and 2) Determine the effect of otolith stimulation during centrifugation on cerebral blood flow in labyrinthine defective inpatients and healthy controls. This aim will be accomplished by varying the radius of rotation of labyrinthine-defective patients and healthy gender- and age-matched controls on a short arm centrifuge. The results of these studies will provide direct evidence on the role of vestibular inputs in cerebrovascular regulation. This work may lead to new methods to diagnose those at risk for falls, as well as ways to train the vestibular system to minimize cerebral hypo-perfusion, syncope, and falls in the elderly. [unreadable] [unreadable]